This invention relates to mobile offshore dwelling units (MODUs), and more particularly to MODU jacking systems, apparatus and methods.
Offshore structures are not unknown. In 1955 the U.S. Army Corps. of Engineers constructed radar stations along the New England coast, which were commonly referred to as xe2x80x9cTexas Towers.xe2x80x9d In constructing these radar stations, the radar platforms were lifted on supporting legs, using hydraulic cylinders. While the legs and the platform were pinned together, a plurality of hydraulic cylinders were manually attached between the supporting legs and the platform. The pins holding the platform stationary with respect to the legs were removed, and the hydraulic cylinders were then pressurized to extend their pistons and raise the radar platform. At the end of the pistons"" strokes, the pins holding the platform in position with respect to the supporting legs were manually replaced to hold the platform in a stationary position with respect to the legs so the plurality of cylinders could be disconnected from the platform and the legs, and their pistons could be retracted without affecting the relative positions of the platform and the legs. The plurality of hydraulic cylinders were then manually reattached between the platform and the legs, and the pins holding the platform stationary with respect to the legs were manually removed, and the hydraulic cylinders were operated again to extend their pistons and raise the platform with respect to the legs. This procedure was repeated again and again until the platform was lifted to its desired position with respect to the plurality of legs. This method of construction was labor-intensive, slow, and expensive.
The increasing need for oil and gas has led to offshore exploration, requiring drilling into the earth""s surface far below the water. Such drilling operations are accomplished from mobile offshore drilling units (MODUs). MODUs generally comprise submersible, semi-submersible and jack-up types, with which the invention is concerned. Jack-up MODUs are massive structures which can have platform surface areas as large as two acres to support the drilling equipment, drilling supplies, power sources, living quarters, helicopter landing ports, and the stores and fuel that are necessary to maintain a drilling crew and operate the MODU and its drilling equipment hundreds of feet above the underwater surface. Jack-up MODUs include a plurality of MODU supporting legs, most generally three legs, that are moveably engaged with the MODU platform. Following their construction, such MODUs, with their MODU platforms resting on footings at the base of each supporting leg are towed to an offshore drilling site, like a large vessel with three 700 foot masts. Once the MODU is positioned at a drilling site offshore, the MODU supporting legs are lowered to engage the earth""s underwater surface and thereafter lift, or jack-up, the MODU platform sufficiently above the water level to reduce exposure of the MODU platform to wave action during severe storms. It is not uncommon for jack-up MODUs to weigh 30,000 to 40,000 tons, or more, with the MODU platform and its variable loads comprising as much as two-thirds of the weight. In addition, it is not uncommon for the MODU supporting legs to have lengths of 600 to 700 feet, and, to provide stability in their support of the MODU platform, to have cross sections, most commonly triangular, up to 50 feet on a side.
The jack-up MODUs currently in use and being constructed include, as the apparatus to adjust the relative position of the MODU platform and MODU supporting legs, a plurality of motor-driven spur gears which engage toothed racks running the length of each corner leg chord of each MODU supporting leg. The leg chords that comprise the corners of the MODU supporting legs of such currently existing jack-up MODUs are constructed with a central toothed rack, of expensive high strength (e.g., 100 KSI) steel, running the length of the supporting leg, with rigidifying semi-circular, tubular structural members welded along both sides of the toothed rack to increase the strength, section modulus and rigidity of the leg chords. Because the spur gears rotationally engage the toothed racks of the leg chords in raising and lowering the MODU supporting legs with respect to the MODU platform, the spur gear teeth and the teeth of the leg chord racks have cycloidal cross sections, and the spur gear drives are each engaged with the leg chord racks by line contact between a single tooth of the spur gear and a single mating tooth of a toothed rack, exposing the teeth of both the spur gear and the rack to extremely high shear forces and requiring that the spur gears and the toothed rack be made of an expensive high-grade steel, with a modulus of elasticity, for example, of 100,000 pounds per square inch (100 KSI).
Because of the great weights being handled and the high stress engagement between the spur gear teeth and rack teeth, as many as 18 spur gear drive units may be engaged with the six toothed racks on each supporting leg. In such systems, the plural spur gear drives are mounted vertically in sets of three units, one above another, so their pinion gears can engage the toothed racks that comprise the leg chords; however, the load is unequally shared by the plurality of engaged pinion gears, the lowest pinion gear and its engaged rack tooth carrying a significantly disproportionate portion of the load. Because the tooth loading in current spur gear driven jack-up MODUs is approaching the stress and fatigue limits of the available materials, complex controls for the electric motors of the spur gear drives have been developed in an effort to equalize the loads that are borne by the plurality of engaged gears and the associated stresses and fatigue. Such controls control the torques generated by the electric motors to balance the loads on their pinion gears and gradually accelerate and decelerate in an effort to avoid overstressing and fatiguing the engaged teeth. Further, during operation of the spur gear drives, grease must be mopped onto the rack teeth by the MODU crew to reduce the friction between the pinion gears and the leg chord racks, and the grease inevitably falls into the sea.
In addition to requiring expensive controls, materials and manufacturing procedures, spur gear-driven jack-up MODUs also require expensive separate locking apparatus for each supporting leg to maintain the MODU platform in a stationary position with respect to its supporting legs.
The jacking systems of jack-up MODUs are currently expensive to design and manufacture and are not expected to satisfy future requirements. There is an increasing demand for larger jack-up MODUs with dramatically greater topside loads. The ability to meet this demand has, however, approached its practical limit with existing materials and technology, and a new jack-up MODU and MODU jacking system are needed.
The invention provides a new jack-up MODU and MODU jacking system that can reliably handle loads several times greater than can be currently handled, can be readily and inexpensively designed and scaled for different jack-up loads, and can save millions of dollars in the manufacture of a single jack-up MODU.
In one aspect of the invention, a plurality of MODU-carried continuous linear motion motors are engaged with a plurality of MODU supporting legs to provide relative motion between the MODU platform and its supporting legs, and to also maintain the MODU platform and MODU supporting legs locked in a stationary relationship. As used herein, the term xe2x80x9ccontinuous linear motion motor,xe2x80x9d refers to a plurality of hydraulic piston/cylinder units N whose piston operations are phased so that Nxe2x88x921 of the plurality of piston/cylinder units are engaged with a MODU-supporting leg and providing relative motion while one of the piston/cylinder units is disengaged from the MODU-supporting leg and being repositioned for re-engagement with the supporting leg to continue the relative motion. The invention thus permits a MODU platform to be automatically jacked up hydraulically with continuous motion, avoiding the excess forces needed to overcome static friction and to accelerate the heavy masses of the MODU.
In the invention, a plurality of hydraulic piston/cylinder units are used to provide continuous relative motion of the MODU with respect to a plurality of MODU-supporting legs that carry a plurality of toothed racks, by phased operation of their pistons, that is, by sequentially engaging different groups of the piston/cylinder units with the plurality of toothed racks and driving their pistons with hydraulic pressure, while another group of the piston/cylinder units are disengaged from the toothed racks and are repositioned for reengagement by application of hydraulic pressure to the cylinders of the disengaged pistons. The pluralities of hydraulic piston/cylinders in their phased operations provide a plurality of continuous linear motion motors that can be controlled from the MODU to jack the MODU up or down, or to lock the MODU in any stationary position. Such a plurality of continuous linear motion motors are substantially less expensive than a comparable plurality of spur gear drives.
In the invention, a multiplicity of teeth are engaged in providing relative motion (and in lifting the MODU platform) at any given moment of time, eliminating high tooth stress by spreading the load imposed by the large weight of the MODU over the multiplicity of teeth provided by a plurality of toothed rack engagement members driven by the plurality of pistons. Furthermore, in the invention, the teeth of the rack engagement members being driven by the pistons of the hydraulic cylinders, and the teeth of the plurality of racks being driven thereby are formed with substantially planar engagement surfaces that spread the stresses from the driving forces uniformly over and through the engaged teeth, and the substantially planar engagement surfaces of the engaged teeth are preferably angled to be normal to the central axes of the plurality of pistons within the central portion of the pistons"" movements.
In another aspect, the invention eliminates the large forces acting transversely on the toothed racks of the leg chords of the supporting legs in the prior art spur-gear driven jack-up systems and eliminates the solid toothed racks of expensive, high modulus (e.g., 100 KSI), steel that extend centrally through each leg chord and provides, instead, a leg chord comprising tubular columns with one or more toothed racks of a steel with significantly reduced modulus of elasticity (e.g., 34-58 KSI) welded on their sides, permitting the jack-up leg chords to be reconfigured to have equal or greater section modulus with less cross-sectional area, permitting huge weight and cost savings.
These features eliminate the requirement to use special high-tensile strength (e.g., 100 KSI) steels in the toothed racks and in the plurality of piston-driven rack engagement members. In addition, where the plurality of piston/cylinder units are pivotally mounted to the MODU, the angled substantially planar engagement surfaces of the teeth generate forces resisting the disengagement of the engaged teeth of the rack engagement members and toothed racks when the pistons are substantially retracted within their cylinders to assist in locking the MODU in a stationary position, and the angled substantially planar engagement surfaces of the engaged teeth of the rack engagement members and toothed racks generate forces assisting the disengagement of the teeth for repositioning of the rack engagement members at the end of the pistons"" stroke.
In the invention, the plurality of driving piston/cylinder units, for at least each leg, are subjected to the same hydraulic pressure when providing relative motion between the MODU and its supporting legs, and any restriction to movement that may result in the exertion of increased pressure on one set of teeth results in increased pressure on all of the acting cylinders, thereby overcoming the restriction to movement without an excessive and unequal force being exerted against any set of teeth.
As indicated above, the invention further includes a locking mode wherein all of the pistons of the plurality of piston/cylinder units are retracted substantially entirely within their cylinders, with their attached toothed rack engagement members engaged with the toothed racks, and providing, in their engagement, forces resisting their disengagement. The locking mode of operation eliminates the expensive separate locking apparatus for each supporting leg that are necessary in current spur gear driven jack-up systems.
Methods of the invention include:
A method of jacking a MODU without interruption, comprising: providing a plurality of MODU supporting legs; providing a plurality of toothed racks fastened to said plurality of MODU supporting legs; providing a plurality of hydraulic piston/cylinder units attached to said MODU, each of said plurality of hydraulic piston/cylinder units having a toothed rack engagement member attached to and driven in a vertical direction by its piston and engageable with one of said toothed racks; engaging a portion of the plurality of said toothed rack engagement members of a portion of said plurality of piston/cylinder units with said toothed racks; and driving said engaged portion of the plurality of toothed rack engagement member by applying hydraulic pressure to said pistons of said portion of the plurality of piston/cylinder units to extend the pistons and thereby continuously provide relative motion between the MODU and MODU supporting legs while a remainder of the toothed rack engagement members are disengaged from the toothed racks and are being repositioned for re-engagement by applying hydraulic pressure to retract their pistons and thereafter for driving the toothed racks.
A method of locking the MODU in a stationary position, comprising disengaging the toothed rack engagement members of a portion of the plurality of piston/cylinder units from the toothed racks; retracting their pistons substantially entirely within the cylinders of the piston/cylinder units and re-engaging the retracted toothed rack engagement members of said portion of the piston/cylinder units while maintaining engagement of the remainder of the toothed rack engagement members with the toothed racks; and repeating the operation with different portions of the toothed rack engagement members of the plurality of piston/cylinder units until all pistons of the plurality of piston/cylinder units are substantially entirely within their cylinders with all toothed rack engagement members engaged with the toothed racks.
A method of manufacturing a MODU jacking system capable of withstanding at least a maximum leg load of W, comprising: manufacturing a plurality of MODU supporting legs capable of carrying a plurality of toothed racks; selecting a number of toothed racks R and fastening the toothed racks on the plurality of MODU supporting legs; and selecting a number of hydraulic piston/cylinders N, having commercially available diameters d; manufacturing a plurality of rack engagement members capable of engagement with the toothed racks and attaching a rack engagement member to each piston of each hydraulic piston/cylinder; providing a source of hydraulic pressure P on the MODU to provide relative motion between the MODU and the MODU supporting legs by application of hydraulic pressure to the hydraulic piston/cylinders; and fastening said plurality of hydraulic piston/cylinder units to the MODU in a manner permitting engagement of their rack engagement members with the toothed racks, said selection of the number R of toothed racks, the number N of hydraulic piston/cylinders per rack, and the diameter d of the pistons being defined by             π      ⁢              xe2x80x83            ⁢                        PRd          ⁢                      xe2x80x83                          2            ⁢              (                  N          -          1                )              4    ≥  W
Further inventive features and combinations are presented in the drawings and more detailed descriptions of the invention that follow.